The use of large fabric bags for transporting semi-bulk quantities of powdered or granular material, bags which can be lifted only by forklift trucks, cranes, or other forms of material handling equipment having hooks or tines, is prevalent. Technically, such bags are known as FIBCs, for "Flexible Intermediate Bulk Containers"; in the industry, they are also known as "bulk bags" and are referred to as such in this specification. The size of a bulk bag may vary substantially, depending upon the density and other characteristics of the material transported, the rated weight capacity of the bulk bag, and other factors. In general, a bulk bag usually has a height substantially exceeding twenty inches (51 cm) and a periphery of more than ninety inches (228 cm). A bulk bag is most often square in cross section, but it may be round or of other cross-sectional configuration.
Early bulk bags are shown in U.S. Pat. No. 4,010,784 of Frank and Peter Nattrass; those bulk bags have four lifting loops attached to the top corners of the bag. The fabric at the top of each corner of the bag is folded into a substantially S-shaped configuration to provide three overlying layers of fabric, and a leg of each lifting loop is inserted between two of the three layers. The resulting assembly is stitched together to secure each lifting loop to the top of the bag. This construction is relatively strong and was commercially successful, but had the disadvantage that the top of the bag was smaller than the lower part of the bag.
Another bulk bag construction, in commercial use in the United States since 1980, is disclosed in U.S. Pat. No. 4,307,764 to Peter Nattrass. That bulk bag utilizes three rectangular fabric members; one forms two sides and the bottom of the bag, whereas the other two members form opposed sides of the bulk bag. Each of these three members has two side edges folded over, affording two upwardly open pockets at each corner of the bulk bag. The legs of the lifting loops are usually inserted into the tops of the two pockets of side wall fabric at each corner of the bag and then sewn in place by multiple lines of stitching. Criss-cross stitching over the height of the lifting loop legs is the preferred construction but other sewing variations have been employed.
Another known bulk bag construction, described in Futerman U.S. Pat. No. 4,362,199, has spread lifting loops at each corner of the top of the bag and utilizes reinforced vertical bands in the side walls of the bag; one of the reinforced bands is aligned with each lifting loop leg. The individual lifting loops are separately stitched to the top of the bag, usually by box stitching. In the preferred construction the top of each side wall is folded over so that two layers of side wall fabric are joined to each lifting loop leg. A variation of this construction is included in Derby et al. U.S. Pat. No. 4,457,456, in which the lifting loops are formed from a continuous length of webbing that extends around the top of the bag; that webbing forms a series of V-shaped projections, one above the top portion of each side wall of the bag. Again, the top of the bag is usually folded over to provide a double layer. A somewhat similar construction is shown in Peter Nattrass U.S. Pat. No. 4,646,357; it employs individual lift loops having their legs spread out across each corner of the bag. The top of the bag is folded over to form a double horizontal layer into which the leg portions of each lift loop are sewn.
Another bulk bag construction is shown in Peter Nattrass U.S. Pat. No. 4,822,179. In the bulk bags shown in that patent, the lifting loops are formed of multiple layers of the same fabric as used in constructing the body of the bulk bag. Anchoring of these lifting loops to the top of the bag is provided by multiple stitching in patterns preferably similar to those in the earlier U.S. Pat. No. 4,307,764.
In a more recent patent, Hughes U.S. No. 5,108,196, the top of a bulk bag is folded over to afford a double layer at the top of the bag side wall. Indeed, a triple layer is described as preferred. In the bulk bags shown in this patent, the leg portions of the lifting loops are aligned with reinforced bands in the side walls of the bag. The lifting loops themselves are secured to the top of the bag by chain stitching that extends around the entire top periphery of the bag, with the lifting loops being anchored into the horizontally folded portion of the bag top. A related construction, but with lifting loops individually attached to the reinforcing strips, is described in the British Patent Publication No. 2 132 171 of S. Hartman. In another bulk bag construction, in F. Nattrass U.S. Pat. No. 4,610,028, the lifting loops are formed as integral extensions of reinforced portions of the side walls of the bulk bag.
A common concept, incorporated in many previously known bulk bags, is that the anchoring of lift loops to plural layers of side wall fabric is desirable to increase the overall strength of the bulk bag when subjected to the various tests utilized in the industry, particularly those tests that are aimed at determining the lifting capacity of the bulk bag; a 5:1 ratio of actual capacity to rated capacity is considered standard. Another prevalent concept, present in some known bags but not in all of them, is that when individual lift loops are utilized they should be anchored to the side wall fabric by individual stitching, either by box stitching or by multiple lines of stitching extending across both legs of a lift loop. The end result of adherence to these two concepts has been and is a relatively high cost of construction for the bulk bags. That high cost arises from two sources: the labor involved in individual stitching of lift loops to side walls and the excess fabric used in the side walls. The labor cost predominates. The present invention is based upon the discovery that these costs can both be materially reduced without appreciable loss of strength in the finished product. In practice, a reduction in labor cost by a factor of four to eight or more is achieved. Thus, it has been ascertained that bulk bags can be constructed with a major reduction in cost while maintaining high standards of strength for the finished bulk bags by utilizing the present invention.